[FFmpeg-user] Swap fields in interlaced video

[Carla Schneider]

This is not a question about top field first or bottom field first, the problem
is already present when there is no motion in the video at all.
Odd and even lines are swapped. The source of the video is
a very old mjpeg capture card.

The video is going to be processed with the commandline tool ffmpeg
Is it possible to swap it back before further processing ?
I could not find anything in the options.